The goal of Project 2 is to study the efficacy and safety of hematopoietic stem cell (HSC) gene therapy for severe combined immunodeficiency (SC1D-X1) using FV vectors in the canine model of SCID-X1. Studies in human patients have shown that insertional mutagenesis is a substantial concern in the treatment of SCID-X1 with HSC gene therapy. The canine SCID-X1 model is an outstanding large animal model to test novel gene therapy and transplantation strategies for the treatment of SC1D-X1. The canine X-linked SCID syndrome, just as in humans, is caused by mutations in the common gamma subunit (?c) of the IL-2, IL-4, IL-7, IL-9 and lL-15 receptors. As in humans, neonatal dogs with SCID-X1 have few peripheral T cells, and the number of peripheral B cells is increased. B cells in canine SCID-X1 are able to produce IgM but are not capable of class-switching to IgG antibodies. The canine SCID-X1 model has been extensively used to study bone marrow transplantation and gene therapy strategies. A major strength of canine studies is the ability to perform long-term evaluations of efficacy and safety. Here we propose to study novel FV vectors and nonmyeloablative conditioning to improve efficacy and safety of gene therapy for SCID-X1. We hypothesize that FV vectors will provide a safer integration site profile in SCID-X1 dogs similar to our preliminary data in normal dogs. We further hypothesize that novel nonmyeloablative conditioning regimens will improve engraftment of gene-corrected HSCs and thus improve long-term immune reconstitution. Furthermore, we test the hypothesis that in vivo administration of FV vectors may improve immune reconstitution. Project 2 will interact closely with all other projects and cores. Specifically, Project 2 will closely work with Drs. Rawlings and Scharenberg who will evaluate FV vectors in the mouse model in Project 1. We will also closely work with Dr. Trobridge, Project Leader of Project 3, to evaluate novel integration site analyses and potentially improved, insulated FV vectors. We will utilize all cores. Core A will facilitate communication among the different projects and cores, Core B will provide all FV vectors. Core C will assist with studies to evaluate immune reconstitution in SCID-X1 dogs, and Core D will assist with the FV vector integration site analyses.